Catalytic dewaxing of hydrocarbon oils to reduce the temperature at which separation of waxy hydrocarbons occurs is a known process and is described, for example, in the Oil and Gas Journal, Jan. 6, 1975, pages 69-73. A number of patents have also described catalytic dewaxing processes, for example, U.S. Pat. No. Re. 28,398 describes a process for catalytic dewaxing with a catalyst comprising a zeolite of the ZSM-5 type and a hydrogenation/dehydrogenation component. A process for hydrodewaxing a gas oil with a ZSM-5 type catalyst is also described in U.S. Pat. No. 3,956,102. A mordenite catalyst containing a Group VI or a Group VIII metal may be used to dewax a low V.I. distillate from a waxy crude, as described in U.S. Pat. No. 4,100,056. U.S. Pat. No. 3,755,138 describes a process for mild solvent dewaxing to remove high quality wax from a lube stock, which is then catalytically dewaxed to specification pour point.
Catalytic dewaxing processes may be followed by other processing steps such as hydrodesulfurization and denitrogenation in order to improve the qualities of the product. For example, U.S. Pat. No. 3,668,113 describes a catalytic dewaxing process employing a mordenite dewaxing catalyst which is followed by a catalytic hydrodesulfurization step over an alumina-based catalyst. U.S. Pat. No. 3,894,938 describes a hydrodewaxing process using a ZSM-5 type catalyst which is followed by conventional hydrodesulfurization of the dewaxed intermediate.
In catalytic dewaxing processes using shape selective catalysts such as ZSM-5, the waxy components particularly the n-paraffins, are cracked by the zeolite into light gases, such as C.sub.1 and C.sub.3 and some heavier olefinic fragments which remain in the lube oil boiling range. These olefinic fragments are unstable to oxidation so that the hydrodewaxed oil is subsequently hydrogenated over catalyst to saturate the olefins and improve the oxidation stability of the oil. The hydrogenation catalysts generally used are mild hydrogenation catalysts such as CoMo/Al.sub.2 O.sub.3 type. The color of the oil may also be improved in this hydrofinishing.
The waxy components in heavy lube fractions, particularly bright stock, contain not only the normal paraffins, but also slightly branched paraffins and cycloparaffins. In the bright stock, the normal paraffins comprise the so-called microcrystalline wax while the slightly branched paraffins and cycloparaffins comprise so-called petrolatum wax. When a shape selective catalyst such as HZSM-5 is used, the microcrystalline wax cracks much faster than the petroleum wax. As a result, when sufficient microcrystalline wax is cracked (e.g. 99+%) to meet the pour point requirement of say, -7.degree. C., there is still some petrolatum wax left, say, 0.5 to 5%. This small amount of petrolatum wax does not impair pour point specification but it makes the oil fail an overnight cloud point (ONC) test (ASTM D-2500-66).
The overnight cloud point test is conducted by placing the finished oil overnight in a refrigerator set at 5.5.degree. C. (10.degree. F.) above the pour point specified, say -7.degree. (about 20.degree. F.). An oil sample passes the test if it remains clear and bright, but some oils, particularly hydrodewaxed oil become dull due to growth of wax crystals, and fail the test. The oil fails the overnight cloud test as soon as the wax crystals nucleate and grow to sufficient sizes of say, 0.05 to 0.5 microns.
If the severity of the dewaxing is increased significantly, the product can be made to meet the overnight cloud point (ONC) test. For instance, decreasing the product pour point to -23.degree. C. (-10.degree. F.) by increasing temperature, decreasing space velocity, etc., can produce a product that passes the ONC test at -1.degree. C. (30.degree. F.). However, this decrease in pour point leads to increased cost (because of reaction severity) and, particularly, to decreased yield.
It would therefore be desirable to find some way of improving the quality of the catalytically dewaxed product so that it is capable of passing the ONC test without incurring the disadvantages of a higher severity dewaxing and, in particular, to avoid the losses in yield concomitant upon such a treatment.